Alpha not working for big?? images

[Matias-Barrios]

I have a weird issue when trying to add alpha to the background of my game. I have this function :

// Get a texture from a window, a renderer and a path
func GetTexture(w *sdl.Window, r *sdl.Renderer, path string, alpha uint8) *sdl.Texture {
    surface, err := w.GetSurface()
    if err != nil {
        panic(err)
    }
    surface.FillRect(nil, 0)

    surfaceImg, err := img.Load(path)
    if err != nil {
        log.Fatalf("Failed to load PNG: %s\n", err)
        os.Exit(4)
    }
    surfaceImg.SetAlphaMod(alpha)
    textureImg, err := r.CreateTextureFromSurface(surfaceImg)
    if err != nil {
        log.Fatalf("Failed to create texture: %s\n", err)
        os.Exit(5)
    }
    surfaceImg.Free()
    return textureImg
}

The problem is, it seems to make textures transparent only when they are small (??). for instance I can make each small block in my game transparent but it does not seem to affect the background, although both are drawn in the same way.

[veeableful]

Hi @Matias-Barrios, could you please create a very minimal example that demonstrate this issue? That would really help me in figuring out the issue. Thanks!

[Matias-Barrios]

Hi @veeableful . I have created this small repo which only draws a background and a block :+1: https://github.com/Matias-Barrios/transparency_bug

You can clone and play around with these two lines : https://github.com/Matias-Barrios/transparency_bug/blob/master/main.go#L32-L33

You will see that although the transparency of the block changes the background is not affected. Please let me know if this is minimalist enough

[veeableful]

Hi @Matias-Barrios, it seems like the texture needs to set a blend mode via textureImg.SetBlendMode(sdl.BLENDMODE_BLEND) so any image can be made translucent. Otherwise, it seems like the image needs to be saved with RGBA color type (based on quick investigation using fotoforensics.com on the images).

[veeableful]

Oh and thank you very much for the mini example! That was super helpful! I think if I tried to recreate the image, I wouldn't have had the same problem as the image I created for assisting with the investigation already had RGBA color type :smiley:

[Matias-Barrios]

@veeableful understood. Thanks for digging into this. I'll try to add the changes and contact you back in case of issues. Awesome! :)